The report, shared with MarketWatch, states that some of Puerto Rico’s debt may have been issued illegally, allowing the government to potentially declare the bonds invalid and courts to then decide that creditors’ claims are unenforceable. The scope of the audit report, issued by the island’s Public Credit Comprehensive Audit Commission, covers the two most recent full-faith-and-credit debt issues of the commonwealth: Puerto Rico’s 2014 $3.5 billion general-obligation bond offering and a $900 million issuance in 2015 of Tax Refund Anticipation Notes to a syndicate of banks led by J.P Morgan

So government officials break the law by taking out a loan they shouldn't have taken out, and the punishment is that they get to keep the money and not pay it back? This is absolutely absurd. That means that completely innocent third parties are essentially being fined $4.4 billion for the malfeasance of Puerto Rico's government officials. Were the creditors truly innocent? Well, the same report goes on to further criticize the government officials for not telling their creditors that what the government was asking for was illegal

Puerto Rico did not inform bondholders that its constitution forbids it from using debt to finance deficits. That, the commission’s report says suggests “substantive” noncompliance with the letter of the constitution

So in fact, incredibly, the creditors' very innocence is used as part of the proof that the debt was illegal, and thus that creditors should be expropriated.

I thought that this couldn't possibly be the law, except that the Supreme Court has already upheld the same outcome in other cases:

The U.S. Supreme Court has said in the Litchfield v. Ballou case and, more recently, in litigation related to Detroit’s bankruptcy that borrowing above a debt ceiling may allow the issuer to declare debt invalid and, therefore, unpayable. Detroit went to court to invalidate $1.45 billion in certificates of participation, debt issued by two shell companies called “service corporations.” The parties settled before the case went to trial, but, while refusing two initial proposed settlements, the judge stated that Detroit’s argument had “substantial merit” and that the suit would have had a “reasonable likelihood of success.”

This is they type of thing that occurs in banana republics. No honest nation with a strong rule of law operates this way. And what is to prevent other distressed government bodies with limited ethics (e.g. the State of Illinois) from carefully borrowing money in a way that is subtly illegal and then repudiate it a few years later?